A cross section of the abdomen of insects reveals a pericardial sinus near the dorsal cuticle. The dorsal diaphragm can be a thin sheet of muscular tissue, or it can be fenestrated (Fig. 1). In most cases, there are muscles present in the diaphragm, which are called alary muscles because when vitally stained they give the appearance of "wings" projecting laterally from each abdominal segment of the dorsal vessel. The presence of paired alary muscles and paired ostia in each segment of the dorsal vessel in the abdomen reinforces the concept of "chambers" of the dorsal vessel in each abdominal segment.
Although mistakenly sometimes thought to play a key role in heartbeat, the alary muscles are more properly called muscles of the dorsal diaphragm. Whereas the myocardium is specialized to contract rapidly and constantly, the ultrastructure of the alary muscles is compatible with muscles that contract infrequently and slowly, having long sarcomeres and few mitochondria to provide only moderate amounts of energy.
In some insects, such as the tsetse fly and some moths, the alary muscles of the vestigial dorsal diaphragm extend from lateral cuticular attachment to join the dorsal vessel in the abdomen, turn, and travel along the dorsal vessel for some distance. Where this occurs, it is more difficult to determine the role of such alary muscles in the heartbeat.
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